Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production and purification rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Rather than trying to discover much lower-cost processes for nano-tubes, we have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but can be produced in larger quantities and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called “nano-scaled graphene plates (NGPs).” Our invented processes include, as examples, (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent pending, Ser. No. 10/858,814 (Jun. 3, 2004); and (3) Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” US Pat. Pending, Ser. No. 11/509,424 (Aug. 25, 2006). NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate (FIG. 2). Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications.
For instance, the following researchers have pointed out the great potential of using NGPs as a new microelectronic device substrate material or a functional material:    1. K. S. Novoselov, et al., “Electric Field Effect in Atomically Thin Carbon Films,” Science 306 (2004) 666-669.    2. H. B. Heersche, et al., “Bipolar Supercurrent in Graphene,” Nature, 446 (March 2007) 56-59.    3. Y. Zhang, Y-W, Tan, H. L. Stormer and P. Kim, “Experimental Observation of the Quantum Hall Effect and Berry's Phase in Graphene,” Nature, 438 (2005) 201-204.    4. Y. Zhang, J. P. Small, M. E. Amori, and P. Kim, “Electric Field Modulation of Galvanomagnetic Properties of Mesoscopic Graphite,” Phys. Rev. Lett., 94 (2005) 176803.    5. C. Berger, et al., “Ultrathin Epitaxial Graphite: Two-dimensional Electron Gas Properties and a Route toward Graphene-based Nanoelectronics,” J. Phys. Chem. B 108 (2004) 19912-19916.    6. G. H. Chen, W. Weng, D. Wu, C. Wu, J. Lu, P. Wang, X. Chen, “Preparation and Characterization of Graphite Nanosheets from Ultrasonic Powdering Technique,” Carbon, 42 (2004) 753-759.    7. H. Fukushima and L. T. Drzal, “Graphite Nanoplatelets As Reinforcements for Polymers: Structural and Electrical Properties,” Proc. Of the 17th Annual Conf. of the Am. Soc. For Composites, Purdue University, (2003).    8. H. Fukushima, S. H. Lee, and L. T. Drzal, “Graphite Platelet/Nylon Nanocomposites,” Proc. of SPE ANTEC (2004) 1441-1445.    9. W. Zheng, et al, “Transport Behavior of PMMA/Expanded Graphite Nanocomposites,” Polymer, 73 (2002) 6767-6773.    10. A. Yasmin and I. M. Daniel, “Mechanical and Thermal Properties of Graphite Platelet/Epoxy Composites,” Polymer, 45 (2004) 8211-8219.
The NGP material can be used as a nano-scaled reinforcement for a matrix material to obtain a nanocomposite. Advantages of nano-scaled reinforcements in a matrix material include: (1) when nano-scaled fillers are finely dispersed in a polymer matrix, the tremendously high surface area could contribute to polymer chain confinement effects, possibly leading to a higher glass transition temperature, stiffness and strength; (2) nano-scaled fillers provide an extraordinarily zigzagging, tortuous diffusion path that results in enhanced barrier or resistance against permeation of moisture, oxygen, other gases, and liquid chemical agents. Such a tortuous structure also serves as an effective strain energy dissipation mechanism associated with micro-crack propagation in a brittle matrix such as ceramic, glass, or carbon; (3) nano-scaled fillers can also enhance the electrical and thermal conductivities in a polymer, ceramic or glass matrix; and (4) carbon-based nano-scaled fillers have excellent thermal protection properties and, if incorporated in a matrix material, could potentially eliminate the need for a thermal protective layer, for instance, in rocket motor applications.
It may be noted that exfoliated graphite flakes (EGFs) are typically obtained by intercalating natural graphite flakes with strong acids to obtain a graphite intercalation compound (GIC). With a sudden exposure to a temperature in the range of 800-1050° C., the GIC expands by a factor of 30-300 to form a “worm,” which is a collection of exfoliated, but largely unseparated graphite flakes. These EGFs are often re-compressed to obtain flexible graphite sheets that typically have a thickness in the range of 0.125 mm (125 μm)-0.5 mm (500 μm).
It may be further noted that EGFs, if fully separated from one another and having a thickness smaller than 100 nm, are considered as nano-scaled graphene platelets (NGPs). It has been recently recognized by researchers in the field of composites that thin, presumably separated EGFs with an extremely high aspect ratio (length/thickness ratio>100˜1000), lead to a lower percolation threshold (typically 1-4% by weight EGF) for forming an electron-conducting path as compared to a threshold of typically 5-20% for other types of graphite particles. However, at these threshold EGF loadings, the electrical conductivity of the resulting composite, typically in the range of 10−5-10−1 S/cm, is still too low to be used for many engineering applications. For instance, the US Department of Energy (DOE) has set forth a target bulk conductivity of 100 S/cm for composite-based fuel cell bipolar plates.
Conventional EGF composites typically contain many substantially unseparated graphite flakes, many of which are thicker than 100 nm. These composites with a high EGF loading either can not be formed into thin composite plate, can not be molded with mass production techniques, or are simply not processable into useful products. Although one would expect the electrical conductivity of an EGF composite to become higher if the EGF loading is greater (e.g., >20% by weight), no composite containing more than 20% by weight of well-dispersed, fully separated EG flakes has hitherto been reported. A need exists for a cost-effective method of preparing EGF/polymer composites with a high EGF loading.
Thus, it is an object of the present invention to provide a highly conductive, thin-film article comprising NGPs or fully separated EGFs wherein the article has a thickness thinner than 50 μm, but could be as thin as 0.1 μm. The thermal conductivity of the thin-film article is greater than 500 W/mK and, in many cases, greater than 1,000 W/mK.
It is another object of the present invention to provide a thin-film article, preferably in a non-woven mat form, comprising NGPs or fully separated EGFs wherein individual platelets or flakes have a thickness smaller than 10 nm.
It is yet another object of the present invention to provide a highly conductive thin-film article comprising NGPs or fully separated EGFs wherein the in-plane thermal conductivity is greater than 500 W/mK and in plane electrical conductivity is greater than 1,000 S/cm.
It is yet another object of the present invention to provide a composite comprising fully separated graphite platelets that are smaller than 100 nm in thickness (preferably smaller than 10 nm) and wherein the weight fraction of platelets is no less than 75%, preferably no less than 85%.
Still another object of the present invention is to provide a composite comprising at least 75% by weight of fully separated graphite platelets wherein the composite has an electrical conductivity greater than 200 S/cm, preferably greater than 500 S/cm.
A specific object of the present invention is to provide a composite comprising at least 75% by weight of fully separated graphite platelets wherein the composite has a thermal conductivity greater than 400 W/mK, preferably greater than 1,000 W/mK.